Link adaptation

ABSTRACT

The present invention discloses an apparatus and method for adapting a transmission parameter in a transmitting node of a data communication system to the current link quality of a data communication channel. The adapted transmission parameter is selected by the transmitting node from a set of transmission parameters in dependence on a number of successful transmissions. The number of successful transmissions is compared in the transmitting node against one of a first threshold value corresponding to a first state of the transmitting node and a second threshold value corresponding to a second state of the transmitting node. The method comprises in the transmitting node the steps of (a) counting the number of successful transmissions; (b) selecting the adapted transmission parameter (b 1 ) in response to the number of successful transmissions equaling or exceeding the first threshold value when the transmitting node is in the first state, and (b 2 ) in response to the number of successful transmissions equaling or exceeding the second threshold value when the transmitting node is in the second state; and in dependence of the result of a following transmission, operating the transmitting node in one of the first state and the second state.

TECHNICAL FIELD

The present invention is related to an apparatus and method for adaptingtransmission parameters to the current quality of a transmissionchannel. More particularly, the invention allows to adapt a variabledata rate or a packet length or both to the channel conditions in awireless local area network.

BACKGROUND OF THE INVENTION

Wireless local area networks (WLANs) have been designed for datacommunication and have found widespread acceptance and proliferation inthe industry. Two wireless broadband LANs (WLANs) are standardized inthe 5 GHz band, namely IEEE 802.11a and ETSI HIPERLAN/2. The physicallayers of both standards are very similar: they both use a modulationtechnique called “Orthogonal Frequency Division Multiplexing (OFDM)” andcan provide up to 8 different transmission modes with data rates rangingfrom 6 Mbps up to 54 Mbps. This multi-rate capability enables a WLANstation to select a transmission mode which is best appropriate to thecurrent radio channel quality to reach the best performance.

In general, adaptive adjustment of the transmission rate is achieved byhaving a receiver estimating the channel link quality, deriving fromthis estimation the rate to be used in future transmissions, and sendingthis information back to the transmitter. The main issues for anefficient link adaptation mechanism are the determination of theparameters to be used for the link quality estimation, e.g. packet errorrate, signal to noise ratio, received signal strength, carrier tointerference ratio, etc., how to measure them, and how to select theappropriate rate out of the measurement results.

In HIPERLAN/2, it is the responsibility of an Access Point (AP) todynamically select any of the available PHY (physical layer) modes forthe down- and uplink transmissions. A Mobile Terminal (MT) continuouslymeasures the quality of the downlink and suggests a suitable downlinktransmission rate to the AP. For the uplink the AP itself performs thelink quality estimation. The standard however does not specify how thelink quality estimation and the corresponding transmission modeselection are performed. S. Simoens and D. Bartolome describe in theirarticle “Optimum performance of link adaptation in HIPERLAN/2 Networks”,VTC 2001, a method for estimating the SNIR (Signal to Noise plusInterference Ratio) and based on this estimation determining thetransmission rate that would maximize the throughput of an HIPERLAN/2network. Similarly, Z. Lin, G. Malmgren, and J. Torsner studied in theirarticle “System Performance Analysis of Link Adaptation in HiperLAN Type2”, VTC Fall 2000, the performance of the link adaptation of HIPERLAN/2when using a C/I (Carrier to Interference ratio) as link qualityparameter.

The standard IEEE 802.11 only specifies which transmission rates areallowed for which types of MAC (medium-access-control layer) frames, butnot how and when to switch between the permitted rates. Furthermore,there is no signaling mechanism specified which would allow a receiverto inform the transmitter about the quality of the communication channelor the rate to be used. The transmitter can change the rate at any timebetween two consecutive packets, but not in the middle of a sequence ofMAC frames belonging to the same packet. The rate at which a MAC frameis transmitted is coded in the header of the physical layer (theso-called PLCP header) which is sent at a fixed rate (6 Mbps in case ofIEEE 802.11a) supported by all stations. Thus, after having decodedsuccessfully the PLCP header, the receiver switches to the indicatedrate to receive the MAC frame.

Although IEEE 802.11 WLANs are becoming more and more popular, littlehas been published about the rate adaptation techniques that could beapplied to those networks. A. Kamerman and L. Montean describe in“WaveLAN-II: A High-Performance Wireless LAN for the Unlicensed Band”,Bell Labs Technical Journal, Summer 1997, pp. 118-133, a method used inLucent's WaveLAN-II devices. It is basically an automatic method forswitching between two transmission rates, with the high one as thedefault operating rate. The device switches automatically to the lowrate after two consecutive transmission errors and back to the high rateeither after ten successful transmissions or after a time out.

As mentioned above, the IEEE 802.11 standard does not specify how rateswitching should be executed in case of multi-rate PHY layers. It onlyspecifies which rates have to be used for sending which MAC frames. Iteven does not provide any protocol means for a receiver to inform thetransmitter about the actual link quality or the transmission rate to beused. That is why the link adaptation method described by G. Holland et.al. in “A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks”,ACM/IEEE International Conference on Mobile Computing and Networking(MOBICOM'01) Rome, Italy, July 2001, cannot be applied to current IEEE802.11 WLANs, since it is based on the principle that the receiverdetermines the link quality and requests the transmitter to switch to amore appropriate rate.

From the above it follows that there is still a need in the art for animproved and efficient link adaptation method. Moreover, onlyinformation available at the transmitter side should be sufficient toguess whether the actual link quality is improving or worsening.

SUMMARY AND ADVANTAGES OF THE INVENTION

In accordance with the present invention, a method for adapting atransmission parameter in a transmitting node of a data communicationsystem to the current link quality of a data communication channel isprovided. The adapted transmission parameter is selected by thetransmitting node from a set of transmission parameters in dependence ona number of successful transmissions s. The number of successfultransmissions s is compared in the transmitting node against one of afirst value s1 corresponding to a first state of the transmitting nodeand a second value s2 corresponding to a second state of thetransmitting node. The method comprises in the transmitting node thesteps of (a) counting the number of successful transmissions s; (b)selecting the adapted transmission parameter (b1) in response to thenumber of successful transmissions s equaling or exceeding the firstvalue s1 when the transmitting node is in the first state, and (b2) inresponse to the number of successful transmissions s equaling orexceeding the second value s2 when the transmitting node is in thesecond state; and in dependence of the result of a followingtransmission, operating the transmitting node in one of the first stateand the second state. The first value s1 is hereafter also referred toas first threshold value s1 and the second value s2 is hereafter alsoreferred to as second threshold value s2.

Preferably, the second threshold value s2 is larger than the firstthreshold value s1, because then the first state can correspond to alink with fast changing quality and the second state can correspond to alink with slow changing quality.

In an embodiment the method can be used for adapting a variable datarate to the link quality, thereby supporting multiple transmissionrates. Selecting the adapted transmission parameter in step (b) which isalso contemplated as switching to the adapted transmission parameterthen comprises switching to a different data rate. This allows theadaptation of the variable data rate to present channel conditions. In afurther embodiment, the step of selecting the adapted transmissionparameter can further comprise selecting a higher data rate from severaldata rates. Also a packet length different to the length employed beforecan be used. Moreover, the variable data rate, the different packetlengths, or other parameters can be combined. This shows the advantagethat several transmission parameters can be adapted to the respectivechannel conditions.

The step of operating the transmitting node in the second state furthercomprises the transition to the first state in the event of a faultytransmission. This has the advantage that it can be switched directlyfrom the second state to the first state, thereby coping with fastchanging channel conditions.

Setting the first threshold value s1 to 3 and the second threshold values2 to 10 leads to an excellent performance in time-varying channels.

The method can further comprise counting a number of faultytransmissions f and selecting the adapted transmission parameter at athreshold of the number of faulty transmissions f_(T). This has theadvantage that also faulty transmissions are considered and a suitablereaction, e.g. reducing the data rate, can be applied accordingly. Inother words, it can, for example, mean switching to a lower data rateimmediately after one faulty transmission.

Setting the threshold of the number of faulty transmissions f_(T) to 1leads to desirable results.

The method can further comprise selecting the transmission parametersused by a responding node, also referred to as responding receiver orstation. For example, the data rate used by that station is taken intoaccount. This allows to use this rate immediately for furthercommunication and can be done as follows. When the transmitting node orstation, also referred to as transmitter, receives a frame correctlyfrom a peer station, i.e. the receiver, it checks whether that frame wassent with a rate different to the one it uses currently for transmittingframes to that station. If this is the case, the transmitter may updateits transmission rate with the one used by the peer. In an preferredembodiment, the transmitter only updates if the transmission rate usedby the peer is higher.

The method provides basically a dynamic link adaptation mechanism thatcan be implemented in a compatible way with the current IEEE 802.11 MACspecification. Using the mechanism, an IEEE 802.11 compliant transmitteris able to detect whether the quality of a link to a certain destinationis improving or declining, and based on this information to select andswitch to the adapted transmission parameters, respectively.

In general, the link adaptation mechanism employs the fact that thetransmitter does not receive an ACK (acknowledgment) for a data framesent to a certain receiver as an indicator that the quality of the linkto that receiver has worsened and therefore, e.g. a lower transmissionrate should be used for future transmissions to that receiver. On theother hand, if the transmitter succeeds to send multiple data frames toa certain receiver, it assumes that the quality of the link has improvedand therefore, e.g. a higher rate should be used for futuretransmissions.

It is advantageous that the mechanism employs only information availableat the transmitter side to determine whether the actual link quality isimproving or worsening and therefore first does not require theavailability of a feedback channel and second remains conform to thestandards.

This can be achieved by the so-called error recovery procedure definedin the MAC (medium access control) layer of the IEEE 802.11 standard.

The link adaptation method described above can be implemented by havingthe transmitter maintaining for a certain destination MAC address twocounters, one for successful transmissions and one for failedtransmissions. If a frame is successfully transmitted, the successcounter is incremented by one and the failure counter reset to zero;similarly, if a transmission fails, then the failure counter isincremented by one and the success counter reset to zero. If the failurecounter reaches a certain threshold f_(T), then the transmission ratefor the corresponding destination is decreased and the failure counterreset to zero. Similarly, if the success counter reaches a certainthreshold s_(T), i.e. the first threshold value s1 or the secondthreshold value s2, then, for example, the transmission rate isincreased and the success counter reset to zero.

In accordance with another aspect of the present invention, there isprovided an apparatus for adapting a transmission parameter to thecurrent link quality of a data communication channel. The adaptedtransmission parameter is selected from a set of transmission parametersin dependence on a number of successful transmissions s. The number ofsuccessful transmissions s is compared against a first threshold values1 corresponding to a first state of the apparatus or a second thresholdvalue s2 corresponding to a second state of the apparatus. The apparatuscomprises a success counter for counting the number of successfultransmissions. The apparatus further comprises a selecting unit forselecting the adapted transmission parameter in response to the numberof successful transmissions s equaling or exceeding the first thresholdvalue s1 when the apparatus is in the first state and in response to thenumber of successful transmissions s equaling or exceeding the secondthreshold value s2 when the apparatus is in the second state. Moreover,the apparatus comprises a decision unit 14 which in dependence of theresult of a following transmission informs the selecting unit 12 tooperate in the first state or the second state.

Furthermore, the apparatus can comprise a failure counter for counting anumber of faulty transmissions, which allows to react on failures intransmission immediately.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in detail below, byway of example only, with reference to the following schematic drawings.

FIG. 1 shows a schematic illustration of a communication environmentwith a transmitting station and a receiving station.

FIG. 2 shows a schematic illustration of a state transition diagram asit is applicable by the transmitting station.

FIG. 3 shows a schematic illustration of the IEEE 802.11a performance ina time-varying channel.

The drawings are provided for illustrative purpose only and do notnecessarily represent practical examples of the present invention toscale.

DETAILED DESCRIPTION

Although the present invention is applicable in a broad variety oftransmission applications it will be described with the focus put on anapplication to wireless systems, i.e. Wireless Local Area Networks(WLAN), using orthogonal frequency division multiplexing (OFDM) asemployed in the WLAN standards IEEE 802.11a and HIPERLAN/2. Beforeembodiments of the present invention are described, some basics, inaccordance with the present invention, are addressed.

As the invention takes advantage of the so-called error recoveryprocedure defined in the MAC (medium access control) layer of the IEEE802.11 standard, this error recovery procedure is described in moredetail below.

The IEEE 802.11 basic access procedure is a distributed procedure basedon the known Carrier Sense Multiple Access (CSMA) method used inEthernet LANs (local area networks). A station with a pending datapacket has to sense the state of the wireless medium before it cantransmit. If the medium is free longer than a predefined time interval,it can proceed with the transmission. Otherwise it first waits until themedium becomes free, then generates a random backoff time before ittries to transmit to minimize the probability of collision with otherstations. MAC (medium access control) frames are protected againsterrors (due to transmission errors or collisions) by means of a framecheck sequence (FCS) field containing a 32-bit cyclic redundancychecksum (CRC) and of a simple send-and-wait automatic repeat request(ARQ) mechanism. If the receiver of a MAC frame detects a CRC error, theframe is discarded. Otherwise, if a MAC (medium access control) framedoes not contain a CRC error, the receiver waits for a short, predefinedSISF (Short Inter-Frame Space) time and sends an ACK (acknowledge) frameback to the transmitter. If the transmitter does not receive an ACKframe within a specified time, it assumes that the transmitted frame isdisturbed and will resend the frame after a random backoff time. Theprocedure is repeated until the transmitter receives an ACK frame fromthe receiver, or a maximum life time or a maximum number of retries isreached.

Generally, the performance and efficiency of the presented linkadaptation mechanism depends on the thresholds for the number ofsuccessful transmissions s and faulty transmissions f. A successfultransmission is considered as a reception of an ACK frame. In the eventthat no ACK frame is received in due time a faulty transmission isassumed. In particular, a success threshold value s_(T) is representedby a first threshold value s1 that corresponds to a first state H or asecond threshold value s2 that corresponds to a second state L for thenumber of successful transmissions s. A failure threshold value f_(T) isset for the number of faulty transmissions f. The impact of these valuesis considered below.

With the failure threshold value f_(T) of the number of faultytransmissions f one can control how long a transmitter should stay,e.g., at a certain rate before it can assume that the link quality isdegraded so that it should switch to a lower rate. A high value of thefailure threshold value f_(T) may impact the performance negatively, inparticular when the link quality is degrading rapidly. Varioussimulation runs have shown that a good value for the failure thresholdvalue f_(T) is one, i.e. the transmitter should switch immediately to alower rate after a failed transmission, regardless how fast the qualityof the link is changing. The efficiency of a such conservative reaction,even when the quality is changing very slowly or not at all, isexplained by the fact that transmissions at a lower rate always have ahigher success chance, in particular when the quality of the channel hasreally worsened.

The success threshold value s_(T), which can equal the first thresholdvalue s1 or the second threshold value s2, defines the maximum number ofsuccessful transmissions s which the transmitter should achieve beforeit can assume that the link quality has improved so that it shouldswitch, e.g., to the next higher data rate. Simulation results revealthat the efficiency of the link adaptation mechanism is sensitive to thevalue of the success threshold value s_(T) and therefore to its firstthreshold value s1 and its second threshold value s2. With reference toFIG. 3 the throughput of a point-to-point link can be represented as afunction of the so-called Doppler spread, which is defined as themaximum frequency at which the channel conditions are changing. LowDoppler spread values correspond to links with slowly changing qualityand high Doppler spread values correspond to links with fast changingquality. When the link quality is changing slowly, a large value for thesuccess threshold value s_(T) leads to a better throughput performance.However, it has been recognized that with a large value for the successthreshold value s_(T) the transmitter does not react fast enough to afast improvement of the link quality. The transmitter is still at a lowtransmission rate although the quality of the link already allows theuse of a higher rate.

With reference to FIG. 1, a general layout of a communication system 8is described in which the adaptation of a transmission parameter in atransmitting node 1 to the current ink quality of a data communicationchannel 7 can be used. As indicated in FIG. 1, a signal can betransmitted via the channel 7. Usually, the signal comprises a frame orseveral frames. The presented embodiment relates to the IEEE 802.11astandard supporting an Orthogonal Frequency Division Multiplexing (OFDM)transmission scheme in the 5 GHz band with variable data rates, i.e. 6Mbit/s to 54 Mbit/s. FIG. 1 shows the transmitting node 1, hereafterreferred to as transmitter 1, and a receiving or responding node 2,hereafter referred to as receiver 2. The transmitter 1 is located at afirst location while the receiver 2 is located at a second location.Multiple of the receiver 2 can be arranged (not shown) within a WLAN.The transmitter 1 comprises a first transmit antenna 3 over which asignal, hereafter called sent signal, is transmitted and a firstreception antenna 4 with which an ACK (acknowledgment) signal, but alsofurther data, is receivable. Both antennas 3, 4 also can form a unit.The transmitter 1 comprises a success counter 10 connected to aselecting unit 12, which further is connected to a decision unit 14. Thetransmitter 1 further comprises a failure counter (not depicted), whichcan be combined with the success counter 10. The success counter 10counts the number of successful transmissions s whenever one ACK(acknowledgment) signal is received via the first reception antenna 4,because then the sent signal was received by the receiver 2 andacknowledged. The selecting unit 12 gets the number of successfultransmissions s from the success counter 10 and switches to an adaptedtransmission parameter accordingly, as described in more detail below.The adapted transmission parameter can be a different data rate, packetlength, or a combination thereof. A set of or multiple differenttransmission parameters can be provided and used. The decision unit 14informs the selecting unit 12 in dependence of the result of asubsequent or following transmission which state the selecting unit 12should use for its further processing. The selecting unit 12 and thedecision unit 14 can form a unity. The method of working in theselecting unit 12 is described in more detail with reference to FIG. 2.

The receiver 2 comprises a second reception antenna 5 with which thementioned sent signal or data is received. A second transmit antenna 6is used to send the ACK (acknowledgment) signal out if valid data hasbeen received.

FIG. 2 shows a schematic illustration of a state transition diagramindicating the mechanism as it is applicable by the transmitter 1 in theselecting unit 12. The mechanism allows to estimate qualitatively thechanging speed of the link quality and to switch dynamically between afirst value s1, also referred to as first threshold value s1, thatcorresponds to a first state, labeled with H, and a second value s2,also referred to as second threshold value s2, that corresponds to asecond state, labeled with L, with s1<s2, depending on whether one is inthe region of high Doppler spread values, i.e. first state H with s1depicted on the left-hand side, or in the region of low spread values,i.e. the second state L with s2 depicted on the right-hand side. Thestate transition diagram in FIG. 2 indicates three states, the firststate H, the second state L, and an intermediate state, labeled with“ACK ?” and depicted above the first and second states H, L in themiddle. The states are connected via arrows which represent thetransition from one to another state or remaining in one state. Thetransition conditions are labeled accordingly and expressed as follows:

failed:

-   -   s:=0, f+, and    -   if f≧f_(T), then down rate and f:=0        means when a transmission failed setting the success counter 10        to zero, incrementing the failure counter and when the number of        faulty transmissions f equals at least the threshold of the        number of faulty transmissions f_(T), then reducing the data        rate and setting the failure counter to zero, or        success:    -   s+, f:=0, and    -   if in state H: s≧s1 or if in state L: s≧s2, then up rate and        s:=0        means when a transmission was successful incrementing the        success counter 10, setting the failure counter to zero and,        when in the first state H the number of successful transmissions        s equals or is larger than the first threshold value s1 or when        in the second state L the number of successful transmissions s        equals or is larger than the second threshold value s2, then        increasing the data rate and setting the success counter to        zero.

The thick arrow lines indicate the switching to an adapted transmissionparameter, e.g. a higher data rate.

In a preferred embodiment the first threshold value s1 equals 3, thesecond threshold value s2 equals 10, and the threshold of the number offaulty transmissions ft equals 1.

The mechanism operates as follows. If the number of successfultransmissions s equals at least to the first threshold value s1 or thesecond threshold value s2, then a selection of and switching to anadapted transmission parameter, e.g. a higher data rate, and atransition to the intermediate state “ACK ?” is performed. In theintermediate state “ACK ?” it is waited for the result of the nexttransmission. In dependence of the result of the next transmission, thefirst state H or the second state L is used.

If the next transmission succeeds, then it can be assumed that the linkquality of the channel 7 is improving rapidly, i.e. high Doppler spread.Therefore, it is moved to the first state H and the success thresholdvalue s_(T) is set equal to the small first threshold value s1 in orderto react quickly to the changing link quality.

If however the next transmission fails, then it is assumed that the linkquality of the channel 7 is either changing slowly or not changing atall, i.e. low Doppler spread, and that the former decision to switch toa higher rate was premature. Consequently, it is moved to the secondstate L and the success threshold value s_(T) is set equal to the highersecond threshold value s2.

If in the first state H a faulty transmission occurs, the first state His retained and the success threshold value s_(T) remains equal to s1 asindicated in the figure. However if in the second state L a faultytransmission occurs, it is moved to the first state H and the successthreshold value s_(T) is changed to the first threshold value s1.

FIG. 3 shows a schematic illustration of the throughput performance ofan IEEE 802.11a WLAN in a time-varying channel. In more detail, theillustration indicates the throughput of a point-to-point link as afunction of the Doppler spread at various values of s_(T), the thresholdof the number of successful transmissions. The two transmission nodesare located 25 m apart and have both the same transmission power of 10dBm. A frequency-flat channel with Rayleigh fading is considered. Anoptimal graph, corresponding to an idealized system where thetransmitters have perfect channel knowledge, is shown as a thick blackline, and runs at about 22 Mbps. Another nearly straight graph at about17 Mbps, indicates a fixed transmission rate of 36 Mbps, which is theone achieving the best results without rate adaptation. The dashed linewith s_(T)=10, f_(T)=1 indicating a first simple adaptive mechanismshows a rapid throughput degradation at high Doppler spreads. The dashedline with s1, s2, f_(T)=1 indicating the adaptive link mechanism whichtakes into account higher as well as lower Doppler spreads shows abetter performance than the dashed line with s_(T)=3, f_(T)=1 indicatinga second simple adaptive mechanism which takes more care to higherDoppler spreads. The dotted lines indicate the throughput achievablewhen the adaptive mechanisms use additionally the data rate of areceived packet from the remote station or receiver. Low Doppler spreadvalues correspond to links with slow changing quality and high Dopplerspread values correspond to links with fast changing quality. Thethreshold of the number of faulty transmissions f_(T) is set for allregarded performance curves to 1. When the link quality is changingslowly, a large value of s_(T) leads to a better throughput performance,see for example the curve for s_(T)=10; however, the performancedegrades rapidly with increasing Doppler spread. With a large value fors_(T) the mechanism does not react fast enough to a fast improvement ofthe link quality. The transmitter is still at a low transmission ratealthough the quality of link already allows the use of a higher rate.

A small value for the success threshold value s_(T) does improve thethroughput at higher Doppler spread values, it however suffersperformance degradation at low Doppler spread values, see for examplethe curve for s_(T)=3. Since the quality of the channel is changing veryslowly or not changing at all, with s_(T)=3 the transmitter switches tohigher rates too early and therefore fails too often. In general theDoppler spread value of a channel is not known a priori; it also changesdynamically. One possible solution is to measure the spread value in thePHY (physical) layer, which however becomes in praxis very complex.Therefore, the presented link adaptation mechanism allows in an easierway to estimate qualitatively the changing speed of the link quality andto switch dynamically between two values of the success threshold values_(T), namely the first threshold value s1 and the second thresholdvalue s2, with s1<s2, depending on whether one is in the region of highDoppler spread values, i.e. the first state H, or in the region of lowspread values, i.e. second state L.

The present invention can be realized in hardware, software, or as acombination of hardware and software. Any kind of computer system—orother apparatus adapted for carrying out the methods described herein—issuited. A typical combination of hardware and software could be ageneral purpose computer system with a computer program that, when beingloaded and executed, controls the computer system such that it carriesout the methods described herein. The present invention can also beembedded in a computer program product, which comprises all the featuresenabling the implementation of the methods described herein, andwhich—when loaded in a computer system—is able to carry out thesemethods.

Computer program means or computer program in the present context meanany expression, in any language, code or notation, of a set ofinstructions intended to cause a system having an information processingcapability to perform a particular function either directly or aftereither or both of the following a) conversion to another language, codeor notation; b) reproduction in a different material form.

1. A method for adapting a transmission parameter in a transmitting node of a data communication system to the current link quality of a data communication channel, the adapted transmission parameter being selected by the transmitting node from a set of transmission parameters in dependence on a number of successful transmissions the number of successful transmissions being compared in the transmitting node against one of a first value corresponding to a first state of the transmitting node and a second value corresponding to a second state of the transmitting node, the method comprising in the transmitting node the steps of: counting the number of successful transmissions; selecting the adapted transmission parameter in response to the number of successful transmissions equaling or exceeding the first value when the transmitting node is in the first state, and in response to the number of successful transmissions equaling or exceeding the second value when the transmitting node is in the second state; and in dependence of the success or failure of a subsequent transmission, operating the transmitting node in one of the first state and the second state.
 2. Method according to claim 1, wherein the step of operating the transmitting node in the second state further comprises in the event of a faulty transmission transitioning to the first state.
 3. Method according to any preceding claim further comprising setting the first value to 3 and the second value to
 10. 4. Method according to claim 1, further comprising counting a number of faulty transmissions and selecting the adapted transmission parameter in dependence of a threshold of the number of faulty transmissions.
 5. Method according to claim 4, further comprising setting the threshold of the number of faulty transmissions to
 1. 6. Method according to claim 1, further comprising selecting the transmission parameter used by a responding receiver.
 7. Method according to claim 1, wherein the step of selecting the adapted transmission parameter further comprises selecting a different data rate.
 8. Method according to claim 1, wherein the step of selecting the adapted transmission parameter further comprises selecting a packet length different to the length used before.
 9. A computer program comprising program code means for performing the steps of the method of claim 1, when said program is run on a computer.
 10. A computer program product stored on a computer usable medium, comprising computer readable program means for causing a computer to perform the steps of the method of claim
 1. 11. An apparatus for adapting a transmission parameter to the current link quality of a data communication channel, the adapted transmission parameter being selected from a set of transmission parameters in dependence on a number of successful transmissions, the number of successful transmissions being compared against one of a first value corresponding to a first state of the apparatus and a second value corresponding to a second state of the apparatus, the apparatus comprising: a success counter for counting the number of successful transmissions; a selecting unit for selecting the adapted transmission parameter in response to the number of successful transmissions equaling or exceeding the first value when the apparatus is in the first state, and in response to the number of successful transmissions equaling or exceeding the second value when the apparatus is in the second state; and a decision unit which in dependence of the success or failure of a subsequent transmission informs the selecting unit to operate in one of the first state and the second state.
 12. Apparatus according to claim 11, further comprising a failure counter for counting a number of faulty transmissions.
 13. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter in a transmitting node of a data communication system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect in the transmitting node the steps of: counting the number of successful transmissions; selecting the adapted transmission parameter in response to the number of successful transmissions equaling or exceeding the first value when the transmitting node is in the first state, and in response to the number of successful transmissions equaling or exceeding the second value when the transmitting node is in the second state; and in dependence of the success or failure of a subsequent transmission, operating the transmitting node in one of the first state and the second state.
 14. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for adapting a transmission parameter in a transmitting node of a data communication system, said method steps comprising the steps of claim
 1. 15. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter in a transmitting node of a data communication system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect in the transmitting node the steps of claim
 2. 16. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter in a transmitting node of a data communication system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect in the transmitting node the steps of claim
 3. 17. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter in a transmitting node of a data communication system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect in the transmitting node the steps of claim
 4. 18. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter in a transmitting node of a data communication system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect in the transmitting node the steps of claim
 5. 19. A computer program product comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter to the current link quality of a data communication channel, the computer readable program code means in said computer program product comprising computer readable program code means for causing a computer to effect the functions of claim
 11. 20. A computer program product comprising a computer usable medium having computer readable program code means embodied therein for causing adaptation of a transmission parameter to the current link quality of a data communication channel, the computer readable program code means in said computer program product comprising computer readable program code means for causing a computer to effect the functions of claim
 12. 